Kamadhenu is the represantation of cow, which grants all wishes and desires. She is the cow of plenty, which emerged from SAMUDRAMANTHAN (the churning of the ocean).
Worshipping it or keeping it in house means prosperity and knowledge, which of course we all need more than anything else in this world. It is beleived that Lakshmi Kamdhenu saligram bring all the wishes and desire come true according to the scripture.